Although many efforts are now focused on preventing HIV-1 transmission within Africa, there remains very little information regarding the underlying molecular mechanisms that determine transmission outcome. While it is well established that the risk of mother-to-child-transmission increases with higher maternal HIV plasma RNA, a threshold of virus has not been identified above which transmission is absolute, nor has a threshold been identified below which transmission does not occur. We therefore hypothesize that HIV-1 transmission is influenced by a complex array of host cellular cofactors that differentially promote (or protect from) viral transmission by influencing local viral entry and replication. In this new investigator revision, we propose to evaluate molecular correlates of HIV-1 transmission, which we term, "co, actors", to better define determinants for perinatal transmission. We will utilize an excellent specimen bank that has been established for transmission fluids obtained cross-sectionally from drug naive, HIV-1C positive mothers and their infants in Botswana to identify transmission cofactors through RNA expression profiling of maternal specimens in association with perinatal transmission. We will then validate cofactor levels and viral expression levels using real-time PCR. Finally, we will evaluate the influence of identified transmission cofactors on HIV-1 replication, in vitro, using full genome molecular clones for HIV-1C and HIV-1B. These aims are designed to identify and functionally characterize the influence of molecular cofactors on HIV-1 entry and replication, to better define biological determinants for transmission. A key feature of this proposal is the intent to foster collaboration between the Boston University School of Medicine (Boston, MA), the Boston University School of Public Health and the Botswana-Harvard Partnership This collaborative effort is essential to elucidate the molecular mechanisms involved in HIV-1C transmission and should be considered a research priority.